Vice President of Research & Collections, Denver Museum of Nature & Science; Dinosaur paleontologist and science communicator; Author, Dinosaur Odyssey: Fossil Threads in the Web of Life

The Gaia Hypothesis

For my money, the deepest, most beautiful scientific explanation is the Gaia hypothesis, the idea that Earth's physical and biological processes are inextricably interwoven to form a self-regulating system. This notion—the 1965 brainchild of chemist James Lovelock, further co-developed with microbiologist Lynn Margulis—proposes that air (atmosphere), water (hydrosphere), earth (geosphere or pedosphere) and life (biosphere) interact to form a single evolving system capable of maintaining environmental conditions consistent with life. Lovelock initially put forth the Gaia hypothesis to explain how life on Earth has persisted for 4 billion years despite a 30% increase in the Sun's intensity over that same interval.

But how does Gaia work? Lacking a conscious command-and-control system, Lovelock and Margulis demonstrated that Gaia uses feedback loops to track and adjust key environmental parameters. Take oxygen, a highly reactive by-product of life, generated and continually replenished by photosynthetic algae and plants. The present day atmospheric concentration of oxygen is about 21%. A few percentage points lower and air-breathing life forms could not survive. A few percentage points higher and terrestrial ecosystems would become overly combustible, prone to conflagration. According to the Gaia hypothesis, oxygen-producing organisms have used feedback loops to maintain atmospheric oxygen between these narrow limits for hundreds of millions of years.

Similar arguments, backed by an ever-growing body of research, can be made for other atmospheric constituents, as well as for global surface temperature, oceanic salinity, and other key environmental metrics. Although the Gaia hypothesis highlights cooperation at the scale of the biosphere, researchers have documented multiple examples showing how cooperation at one level could evolve through competition and natural selection at lower levels. Initially criticized by serious scientists as new-age mumbo-jumbo, Lovelock's radical notion has increasingly been incorporated into scientific orthodoxy, and key elements are now often taught as "Earth Systems Science." One timely lesson resulting at least in part from Gaian research is that food web complexity, including higher species diversity, tends to enhance ecological and climate stability.

So, while Earth may inhabit a "Goldilocks zone," neither too close nor too far from the sun, life's rampant success on this "pale blue dot" cannot be ascribed to luck alone. Life has had a direct hand in ensuring its own persistence.

Science has not yet fully embraced the Gaia hypothesis. And it must be admitted that, as an explanation, this idea remains incomplete. The insights cascading from Gaia are unquestionably deep and beautiful, uniting the whole of the biosphere and Earth's surface processes into a single, emergent, self-regulating system

Yet this explanation has yet to achieve the third milestone defined in this year's Edge Annual Question—elegance. The Gaia hypothesis currently lacks the mathematical precision of Einstein's E=Mc2. No unified theory of Earth and Life has been presented to explain why life stabilizes more than it destabilizes.

Evolutionary biologist W. D. Hamilton once compared Lovelock's insights to those of Copernicus, adding that we still await the Newton who will define the laws of this grand, seemingly improbable relationship. Hamilton himself became deeply engrossed in seeking an answer to this question, developing a computer model that seemed to show how stability and productivity could increase in tandem. Were it not for his untimely death, Hamilton might have emerged as that modern-day Newton, becoming, in the words of author Tim Flannery, "the most revered biologist of all time."

The cultural implications of Gaia also continue to be debated. Arguably the most profound implication of Lovelock's idea is that Earth, considered as a whole, possesses many qualities of an organism. But is Gaia actually alive, akin to a single life form, or is it more accurate to think of her as a planet-sized ecosystem? Lynn Margulis argued strongly (and convincingly, to my mind) for the latter view. Margulis, whose work revolutionized evolutionary biology at the smallest and grandest of scales, died recently. Always the hard-nosed scientist, she once said,

"Gaia is a tough bitch—a system that has worked for over three billion years without people. This planet's surface and its atmosphere and environment will continue to evolve long after people and prejudice are gone."

While not disagreeing with this blunt assessment, I find considerably greater inspiration in Gaian thinking. Indeed I would go so far as to suggest that this idea can help shift the human perception of nature. In the modernist perspective, the natural world is little more than a collection of virtually infinite resources available for human exploitation. The Gaian lens encourages us to re-envision Earth-bound nature as an intertwined, finite whole from which we evolved, and in which we remain fully embedded. Here, then, is a deep and beautiful perspective in desperate need of broad dissemination.